A projection-type display device includes an illumination device, a display element that is illuminated by the illumination light from the illumination device, and a projection lens that magnifies and projects the image displayed on the display element onto a screen. The illumination device is made up of a white light source, and a color wheel in which red (R), green (G), and blue (B) color filters are arranged in a disk form. The light from the white light source is entered onto the color wheel that rotates at high speed, whereby an illumination light is obtained in which color switches in a time sequence.
In the projection-type display device that is provided with the above-described illumination device, a full-color image can be displayed on a screen according to the principles of successive additive color mixture by displaying images of these color components on display elements in synchronization with the switching of the illumination light. This type of projection-type display device is referred to as a field-sequential or time-division projection-type display device.
A high-luminance light source such as a high-pressure mercury lamp is used as the white light source. However, a discharge lamp such as a high-pressure mercury lamp, while having high luminance, gives rise to the problems described hereinbelow in an illumination device that is combined with a color wheel or in a projection-type display device that uses such an illumination device.
Such a lamp is inconvenient to use because a lengthy time is required from being turned on until reaching a steady state of brightness, and further, after being turned off, a waiting period is also required for sufficient cooling before the lamp can be relighted.
When colored light is obtained from a white light source by a color wheel, the light utilization efficiency is extremely poor because, for example, blue and green light cannot be used during the time interval in which light passes through the red color filter to give red illumination.
In addition, two colors are mixed when light passes through the boundary portions of the color filters. As a result, the color purity of light that has passed through the boundary portions is degraded, or the light utilization efficiency drops because this light is not used in the projection display.
Still further, the time or order of switching colors is fixed by the color wheel that is employed.
In addition to the above, there is the problem that mechanical parts that causes the color wheel to rotate at high speed and sensors and electronic circuits that control the stability of rotation are required, leading to a corresponding increase in the cost of the device. Still further, there is the problem of noise during high-speed rotation.
Recent years have seen the development of higher luminance of light sources such as light-emitting diodes (LEDs) and semiconductor lasers (LDs) that are referred to as semiconductor light sources or solid-state light sources. The light emitted from these semiconductor light sources has a narrower spectral width than the light of white light sources of which discharge lamps such as high-pressure mercury lamps are representative and have the feature of enabling higher color reproducibility to be obtained without the use of color filters.
If an illumination device is configured using, for example, red (R), green (G), and blue (B) LEDs and a color synthesis optical element in place of a white light source and color wheel and the LEDs of each color are successively lighted, an illumination device is obtained in which colors are switched in a time sequence.
In contrast to a discharge lamp in which time is required for brightness to reach a steady state of brightness after lighting, a semiconductor light source such as an LED obtains an illumination light as well as a projected image that is bright immediately after lighting, and moreover, requires no waiting time for cooling before relighting. As a result, using a semiconductor light source as the light source of a projection-type display device improves user convenience.
In addition, an LED has a longer service life than a discharge lamp and is superior from an environmental standpoint because mercury is not used. The utilization efficiency of light is high because blue and green LEDs are extinguished when red is irradiated by an LED, and lower power consumption can thus be realized. Still further, installing a dimming function that controls the amount of current of the LEDs enables precise power saving according to conditions.
Because each of the red, green, and blue LEDs can be separately controlled, the time and order of color switching can be controlled electronically, colors can be switched freely, and moreover, synchronization with the display elements can be achieved with high accuracy. Because colors can be switched at high speed, color break-up that was problematic in the field-sequential mode color display can be markedly reduced. The ability to obtain not only R-G-B illumination light but also C (cyan), M (magenta), Y (yellow), as well as W (white) illumination light also enables display by a display mode that prioritizes brightness by displaying these color images. Still further, the problems of deterioration of rotation mechanism parts or noise caused by high-speed rotation do not arise.
Due to these many advantages afforded by LEDs, an illumination device that uses, for example, LEDs and a color synthesis optical element in a projection-type display device is highly anticipated.
However, emission light of a sufficient brightness cannot currently be obtained from a single LED. Thus, in order to realize higher luminance, various techniques for combining a plurality of colors have been proposed. For example, Patent Documents 1-3 disclose light source devices that combine luminous flux from a plurality of LEDs having different peak wavelengths by means of dichroic mirrors or dichroic prisms. These light source devices are of a mode in which differences in wavelength are used to synthesize colored light by dichroic mirrors.
Alternatively, light source devices are disclosed in Patent Documents 4 and 5 in which at least one of three light sources is of a configuration in which a plurality of light sources having different peak wavelengths are disposed in array form. This is a mode of spatially synthesizing colored light.
Yet another mode of synthesizing colored light is a technique that uses polarization. For example, an illumination device is disclosed in Patent Document 6 in which light from two light sources that emit light having random polarization directions is converted to linearly polarized light having polarization directions that are mutually orthogonal and is then synthesized by a polarization beam splitter.
As a related invention, Patent Document 7 discloses a light source device in which light of each color is arranged in a specific polarization direction in advance and then synthesized by a dichroic prism. Still further, a projection-type display device is disclosed in Patent Document 8 in which the polarization direction of incident light is selected while taking into consideration the incident angle dependency when colors are synthesized by a dichroic prism.
The color synthesis optical element that is used in the light source device described in Patent Document 8 includes blue-reflecting multilayer film and red-reflecting multilayer film. FIG. 1A shows the spectral reflectance characteristic of the blue-reflecting multilayer film, and FIG. 1B shows the spectral reflectance characteristic of the red-reflecting multilayer film.
As shown in FIG. 1A, the cutoff wavelength of S-polarized light of the blue-reflecting multilayer film is at least 510 nm but no greater than 540 nm. As shown in FIG. 1B, on the other hand, the cutoff wavelength of S-polarized light of the red reflecting multilayer film is at least 540 nm but no greater than 560 nm.
The light (P-polarized light) from a green light valve (display element) is entered into the blue-reflecting multilayer film and the red-reflecting multilayer film, and light (S-polarized light) from red and blue light valves (display elements) is entered into blue-reflecting multilayer film and red-reflecting multilayer film.